The invention relates to fixing of sheaths on glass fibre reinforced plastic rods, particularly where the glass fibre rod constitutes the load bearing core of a composite electric insulator. This invention relates to metal sheaths and also relates to sheaths of any other materials besides metal, so long as the coefficient of expansion of the sheath material differs from that of the glass fibre rod.
A problem arises from application of force by metal sheaths or mountings into glass fibre rods. Because of the generally unidirectional fibre reinforcement of such a rod, both its strength and its elastic and plastic deformability in the longitudinal direction and in the transverse direction are markedly different. Glass fibre reinforced plastic rods also differ from the metal or other materials used in their sheathing in other significant properties.
Various affixation methods are known for obtaining pull proof connections between sheaths or mountings and the ends of glass fibre rods. Thus, German Laid-open Patent Specification (DAS) 1,261,358 and Patent of Addition 1,400,003 describe a method of producing a connection between a metal sheath and a plain rod or tube of plastic material, wherein as the sheath is pressed onto a glass fibre rod, the specific pressing forces decrease from the end of the rod toward the point of its emergence from the sheath. Because of the markedly different elastic elongation and the markedly different elasticity limits of the glass fibre rod and the sheath materials, clamping or gripping lengths equal to six times the diameter of the glass fibre rod are necessary in order to make thorough use of the strength of the rod. With high stresses, relative motion between the highly elastic rod and the sheath cannot, however, be avoided.
Another method for affixing of metal sheaths on glass fibre rods is described in German Laid-Open Pending Patent Application (DOS) 1,921,229. It comprises deforming the ends of the plastic impregnated glass fibre rod and the surrounding parts of the sheath in an undulating fashion. A cap is cemented on the end of the insulator. That end of the insulator is split and is given a conical shape by a wedge. However, the connection of the insulator is disturbed by the cleaving operation and the cleaving impairs its mechanical strength. Moreover, the glass fibre rod may be enclosed at its ends by a bush having a central opening that tapers conically toward the centre of the insulator, and an external thread. To connect the glass fibre rod with this bush, the end of the glass fibre rod that is engaged in the bush is fanned out by driving a conical pin into it, so that the glass fibre rod is applied against the wall of the conical opening of the bush.
Finally, it is also known to provide the sheath with at least one inner and outer adhering surface and to force the glass fibre rod against it by means of an external clamping element. With such compression or clamped joints, disadvantages occur under unfavorable circumstances in that as a result of subsequent shrinkage of the plastic or by reason of the different coefficients of thermal expansion of the metal sheath and the plastic rod, the insulator becomes detached from the sheath.
In all of the above prior arrangements, the tensile strength of the glass fibre rod cannot be fully employed. Fundamentally, all of these wedge shaped and conical connections between the sheath and the plastic rod permit only a very limited utilization of the strength of the rod, since with such rigid connections, only the foremost section of the sheathing is able to transmit forces. Those sections of the sheathing which are located further toward the end of the rod are largely relieved of load by the different elastic properties of the glass fibre rod. Furthermore, all of these forms have the inherent disadvantage that the comparatively highly elastic glass fibre rod is connected directly to the rigid material of the sheath.